To recruit Mercs you can either use the menu at the Mercs Housing within your base, 

or you recruit them via the menu on the left (offense).

To choose the number of your new units, you can use the slider or the input field. Now simply click on “Recruit units” to start the process.

The little digit on the right of the offense-button will show you the current number of units which are available. A red colored digit means that you haven’t reached your unit limit yet, and can still recruit some more. To raise the limit, simply expand the mercs housing.